(1) Industrial Application Field of the Invention
The present invention relates to an electric air-fuel ratio control apparatus in an internal combustion engine. More particularly, the present invention relates to a electric feedback correction control system in which an oxygen concentration in an exhaust gas is detected, the air-fuel ratio of an air-fuel mixture sucked in the engine is detected based on this oxygen concentration and the fuel injection quantity is controlled by feedback correction to bring the actual air-fuel ratio close to the theoretical air-fuel ratio.
(2) Description of the Related Art
A conventional electronically controlled fuel injection apparatus which is provided in an internal combustion engine has an electromagnetic fuel injection valve in an intake system of the engine. Some of the conventional apparatus have been provided by the applicant as U.S. Pat. Nos. 4,615,319, 4,655,188, 4,729,359 and 4,715,344 or U.S. patent application Nos. '87 97,682, '87 98,038 and '88 179,535.
In these electronically controlled fuel injection apparatus, a basic fuel injection quantity T.sub.p (=K.times.Q/N; K is a constant) is calculated from a sucked air flow quantity Q of the engine detected by an air flow meter and an engine revolution number N detected by an engine revolution speed sensor such as a crank angle sensor, and a correction coefficient COEF including various correction coefficients corresponding to engine driving conditions such as an engine temperature, an air-fuel ratio feedback correction coefficient LAMBDA and others are calculated. The basic fuel injection quantity T.sub.p is corrected according to the calculation result to set a final fuel injection quantity Ti(=T.sub.p .times.COEF.times.LAMBDA+Ts). Ts stands for a correction quantity pertaining to a fluctuation of a battery voltage.
A driving pulse signal having a pulse width corresponding to the so-set fuel injection quantity Ti is put out to an electromagnetic fuel-injecting valve at a predetermined timing to inject and supply a preferable amount of a fuel to the engine.
The air-fuel ratio feedback correction coefficient LAMBDA is to control the air-fuel ratio of the air-fuel mixture sucked in the engine to a predetermined target or aimed air-fuel ratio (the theoretical air-fuel ratio), and the value of the air-fuel ratio feedback correction coefficient LAMBDA is changed by the proportion-integration (PI) control to control the air-fuel ratio stably.
More specifically, an oxygen sensor in which the electromotive force abruptly changes at the theoretical oxygen concentration ratio in the exhaust gas, attained on combustion of the air-fuel mixture at the theoretical air-fuel ratio, and the electromotive force is high in case of a rich air-fuel mixture and the electromotive force is low in case of a lean air-fuel mixture (Japanese Unexamined Utility Model Publication No. 61-182846) is disposed in the exhaust system of the engine. The output voltage from this oxygen sensor is compared with a predetermined reference voltage (slice level) and it is judged whether the air-fuel ratio of the air-fuel mixture sucked in the engine is richer or leaner as compared with the theoretical air-fuel ratio. In the case where the air-fuel ratio is lean (rich), the air-fuel ratio feedback correction coefficient is gradually increased (decreased) by predetermined integration quantity (portion I) to increase (or decrease) and correct the fuel injection quantity Ti and accordingly the air-fuel ratio is easily controlled to the theoretical air-fuel ratio.
The air-fuel ratio feedback correction coefficient LAMBDA is thus set based on the rich-lean judgment of the air-fuel ratio detected by the oxygen sensor to bring the actual air-fuel ratio close to the theoretical air-fuel ratio, and if this control is performed, since a ternary catalyst effectively acts at the theoretical air-fuel ratio, good exhaust gas characteristics can be maintained.
In the case where the air-fuel ratio is controlled in the above-mentioned manner by using the oxygen sensor when the air-fuel ratio feedback correction coefficient LAMBDA is changed by the integration control with a predetermined constant integration quantity, a response delay of the control is caused at the time of rich-lean reversion. Namely, when rich (lean)-to-lean (rich) reversion is judged, since a certain deviation from the theoretical air-fuel ratio has already been estimated, if it is intended to restore the theoretical air-fuel ratio in this state by the constant integration control, a long time is required and therefore, the width of the air-fuel ratio controlled by the air-fuel ratio feedback control is increased (FIG. 20).
Accordingly, this response delay should be eliminated by the proportion control. However, abrupt change of the air-fuel ratio feedback correction coefficient LAMBDA by the proportion control cannot be avoided in the prior control system. The air-fuel ratio obtained by changing the air-fuel ratio feedback correction coefficient LAMBDA is changed substantially at the same frequency of the change of the air-fuel ratio feedback correction coefficient LAMBDA under a strong influence of the abrupt change of the air-fuel ratio feedback correction coefficient LAMBDA and by the change (surge) of the output of the engine caused by the change of the air-fuel ratio, a minute horizontal vibration is generated in a vehicle.
In order to prevent this horizontal vibration of the vehicle, it may be necessary to stabilize the combustion by advancing the ignition timing. However, if the ignition timing is advanced, the combustion temperature rises and the content of nitrogen oxides NO.sub.x will increase.
Incidentally, in the afore-mentioned control system for air-fuel ratio feedback correction coefficient, since the output of the oxygen sensor abruptly changes at the theoretical air-fuel ratio(FIG. 7), the theoretical air-fuel ratio can hardly be specified based on the output of the oxygen sensor. In other words, since the electromotive force output from the oxygen sensor corresponding to the theoretical air-fuel ratio is within a certain range, there may be an apprehension that it can be judged whether the air-fuel ratio is rich or lean as compared with the theoretical air-fuel ratio. In order to prevent such a problem, it may be necessary to use a type of oxygen sensor in which the output of which has a value that gradually changes in the vicinity region of the theoretical air-fuel ratio.
On the other hand, in the conventional air-fuel ratio feedback control system, the control is performed only based on the large-small relation of the air-fuel ratio to the target air-fuel ratio as discussed above, and the response characteristic of the actual oxygen sensor to the change of the air-fuel ratio is about 100 ms at highest and when the output voltage of the oxygen sensor crosses the slice level voltage, the actual air-fuel ratio is greatly changed from the target air-fuel ratio to the rich side or the lean side, resulting in insufficient control (hatched region in FIG. 20). Accordingly, the overshoot or undershoot quantity is increased to increase the variation width of the air-fuel ratio and degrade the convergence to the target air-fuel ratio, with the result that the driving characteristic is degraded by the surge torque and the discharge quantities of CO, HC and NO.sub.x are increased.